This invention relates to the time varying signals representing the vibration of a rotating machine and more particularly to the compensation of the bipolar bias in such signals.
In the design of protection and diagnostic electronics associated with a rotating machine such as a turbine generator, it is desirable to capture information contained in the time varying waveform of a signal representing the vibration of the machine. A periodic signal in the form of a pulse train is used to set boundaries for evaluation of the samples. When a periodic signal is not present, a timer with a fixed expiration determines the boundaries for evaluation. The information in the time varying waveform is usually offset with a bias voltage that has an amplitude which is many times greater than the amplitude of the relatively high frequency part of the signal representing the machine vibration.
It is important to capture the constant bias or xe2x80x9coffsetxe2x80x9d voltage as that voltage is considered to be the xe2x80x9cgapxe2x80x9d between the vibration sensing probe and the rotating shaft of the machine. In addition, it is just as important for the protection and diagnostic electronics to analyze the information contained within the relatively low amplitude high frequency portion of the machine vibration signal. Therefore, it is to the advantage of the electronics if as much of the bias can be recorded and removed prior to sampling of the machine vibration signal by the analog-to-digital converter (ADC) in the electronics as this maximizes the ratio of the amplitude of the alternating waveform to the remaining signal.
There are several techniques that have been used in the prior art to remove the offset voltage. One such technique uses a capacitor to only pass the time varying portion of the signal and block the constant part. The disadvantage with this approach is the inability to record the bias value.
Another such technique involves a similar configuration as that of circuit 10 shown in FIG. 1 which as will be described in more detail below is used to practice the method of the present invention, but with digital potentiometers and operational amplifiers in place of the digital-to-analog converter (DAC) 22 used in the present invention. The algorithm associated with the method that uses digital potentiometers and op amps first determines the bias compensation voltage for a fixed number of digital potentiometer settings, then uses the closest value to the bias for compensation. When the lowest setting has been employed, the algorithm records the bias at the input as the amount of compensation applied plus the remaining bias. The advantage of the method of the present invention over this prior art method is that the method of the present invention better maximizes the ratio of high frequency signal to the total signal range applied to the ADC.
A method for removing the bipolar bias level from a time varying signal and recording the level thereof. The method has the step of firstly determining upon the occurrence of a new eventxe2x80x94either a pulse or a timer expirationxe2x80x94if the maximum amplitude of a collection of samples observed between events is equal to a predetermined positive saturation level and multiplying said predetermined positive saturation level by a negative constant which is less than one when said first determination is true to determine a first bias level compensation value.
The method also has the step of secondly determining when the first determination is not true if the minimum amplitude of the collection of samples observed between events is equal to a predetermined negative saturation level and multiplying said predetermined negative saturation level by the negative constant when the second determination is true to determine a second bias level compensation value.
The method has the further steps of thirdly determining if both the first and second determinations are not true the average of the maximum and minimum amplitudes of the collection of samples observed between events and multiplying said average by said negative constant when said average is not within a predetermined range of amplitude values to determine a third bias level compensation value; and adding the first or the second or the third bias level compensation value to a value for the bipolar bias level previously accumulated during the occurrence of the event occurring just prior in time to the new event occurrence.
An apparatus for removing the bipolar bias level from a periodic time varying signal and recording the level thereof. The apparatus has an analog to digital converter having a predetermined sampling rate for converting the time varying signal to digital samples and a digital signal processor. The digital signal processor firstly determines upon the occurrence of a new event if the maximum amplitude of a collection of the digital samples observed between events is equal to a predetermined positive saturation level and multiplies the predetermined positive saturation level by a negative constant, which is less than one, when the first determination is true to determine a first bias level compensation value.
The digital signal processor then secondly determines, when the first determination is not true, if the minimum amplitude of the collection of digital samples observed between events is equal to a predetermined negative saturation level and multiplies the predetermined negative saturation level by the negative constant when the second determination is true to determine a second bias level compensation value.
The digital signal processor then thirdly determines, if both the first and second determinations are not true, the average of the maximum and minimum amplitudes of the collection of digital samples observed between events and multiplies the average by the negative constant when the average is not within a predetermined range of amplitude values to determine a third bias level compensation value.
The digital signal processor then adds the first or the second or the third bias level compensation value to a value for the bipolar bias level previously accumulated during the occurrence of the event occurring just prior in time to the new event occurrence.
An apparatus for removing the bipolar bias level from a periodic time varying signal and recording the level thereof. The apparatus has an analog to digital converter having a predetermined sampling rate for converting the time varying signal to digital samples and a digital signal processor. The digital signal processor determines if a new digital sample has arrived at its input.
The digital signal processor then determines from the amplitude of the new digital sample if either positive or negative saturation has occurred at the digital signal processor input and when positive saturation has occurred replaces the stored maximum amplitude for samples occurring prior to the occurrence of the new digital sample with the new sample amplitude and when negative saturation has occurred replaces the stored minimum amplitude for samples occurring prior to the occurrence of the new digital sample with the new sample amplitude.
The digital signal processor then low pass digitally filters the new digital sample and determines from the amplitude of the digitally filtered new digital sample if the positive or negative amplitude is greater or less than the stored maximum amplitude or stored minimum amplitude for samples occurring prior to the occurrence of the new digital sample and replaces either the stored maximum amplitude when the digitally filtered new digital sample amplitude is greater than the stored maximum amplitude or the stored minimum amplitude when the digitally filtered new digital sample amplitude is less than the stored minimum amplitude.
The digital signal processor then determines if a new event has occurred and upon the occurrence thereof firstly determines if the maximum amplitude stored during the determination from the amplitude of a new digital sample if positive saturation has occurred is equal to a predetermined positive saturation level and multiplies the predetermined positive saturation level by a negative constant which is less than one when the first determination is true to determine a first bias level compensation value.
A method for removing the bipolar bias level from a periodic time varying signal and recording the level thereof. The method determines from the amplitude of a new digital sample of the time varying signal if either positive or negative saturation has occurred at the input of a digital signal processor and when positive saturation has occurred replaces the stored maximum amplitude for samples occurring prior to the occurrence of the new digital sample with the new sample amplitude and when negative saturation has occurred replaces the stored minimum amplitude for samples occurring prior to the occurrence of the new digital sample with the new sample amplitude.
The method then low pass digitally filters the new digital sample and determines from the amplitude of the digitally filtered new digital sample if the positive or negative amplitude is greater or less than the stored maximum amplitude or stored minimum amplitude for samples occurring prior to the occurrence of the new digital sample and replaces either the stored maximum amplitude when the digitally filtered new digital sample amplitude is greater than the stored maximum amplitude or the stored minimum amplitude when the digitally filtered new digital sample amplitude is less than the stored minimum amplitude.
The method then determines if a new event has occurred and upon the occurrence thereof firstly determines if the maximum amplitude stored during the determining from the amplitude of a new digital sample if positive saturation has occurred is equal to a predetermined positive saturation level and multiplies the predetermined positive saturation level by a negative constant which is less than one when the first determination is true to determine a first bias level compensation value.